1. Technical Field
The present invention relates to data transmission. More particularly, the present invention relates to reducing collisions by providing improved channel access synchronization across a network. Specifically, the present invention relates to node synchronization without a time-stamp exchange through adjusting slot timing based on the received preamble of other nodes in the network.
2. Background Information
Time division multiple access (TDMA) is a channel access method for shared networks. It allows several users or nodes to share the same frequency channel by dividing the signal into different time slots. The users or nodes transmit in rapid succession, one after the other, each using its own time slot. This allows multiple users or nodes to share the same transmission medium (e.g. radio frequency channel) while using only a part of its channel capacity. TDMA is used extensively in satellite systems and combat-net radio systems. However, inasmuch as to combat-net radio systems are typically a mobile ad hoc network, the chaotic environment and ever changing number and distance of users or nodes has presented problems for the standard TDMA channel access method.
Attempts have been made to utilize the channel access methodology of TDMA in mobile ad hoc networks. Mobile ad hoc networks are self-configuring infrastructure-less networks of mobile devices connected by wireless communication, and may include nodes which are greater than one hop from each other. Each device in a mobile ad hoc network is free to move independently in any direction, and will therefore change its links to other devices frequently. Each must forward traffic unrelated to its own use, and therefore also be a router. The primary challenge in building a mobile ad hoc network is equipping each device to continuously maintain the information required to properly route traffic. Such networks may operate by themselves or may be connected to the larger Internet. However, adapting TDMA to a mobile ad hoc network is particularly difficult because the nodes can move around and vary the timing advance required to make its transmission match the gap in transmission from its peers.
Thus, it is both critical and difficult to synchronize time in TDMA mobile ad hoc networks. As discussed above, each node in this network transmits during an assigned time slot of a frame, starting at a given start of slot time (TSOS). TDMA assumes TSOS is synchronized across all nodes in the network as collisions will occur if TSOS is not synchronized among all of the network nodes. However, in the mobile ad hoc network environment, TSOS can become out of sync as nodes move around.
Some prior art systems attempt to address the out-of-sync problem by configuring the receiving nodes to allocate a small listening window immediately following a perceived TSOS boundary for a particular transmitting node. If a preamble is not detected within that receiving window, the receiving node will flag the condition as an error and reset the receiving circuitry. Thus, if the TSOS boundary between the sending node and receiving node is not at least somewhat in sync, communication will not be possible because the receiving node will never be receiving while the transmitting node is transmitting the preamble. The two nodes will continuously miss “hearing” each other. Therefore, these prior art systems are not suitable for mission critical ad hoc mobile networks such as combat-net radio systems due to their unreliability.
Other prior art attempts to address the out-of-sync issue implement a global clock or some universal monotonically increasing value for all the nodes to utilize for their timing. This method requires time stamp or counter values to be exchanged among the nodes, with the nodes adjusting their internal timers accordingly and calculating TSOS therefrom. Such a system requires one leader node to set and distribute the global clock. However, in a mobile ad hoc network, no node is guaranteed to remain in the network. Thus, selecting and maintaining a leader node quickly becomes unmanageable. Further, such a time stamp exchanging method requires a working communication channel and does not handle the case of sub-network merging in which the timing from the merging networks may be so different that successful initial communication is not guaranteed. Thus, the time-stamps never get exchanged and the merging network never communicates with the other network.
Thus, there is a tremendous need in the art to overcome the above stated disadvantages of the prior art for providing channel access synchronization without time-stamp exchange in a TDMA multi-hop and/or mobile ad hoc network.